Page 98 - Radar Technology Encyclopedia

88 clutter, surface CODE, CODING

Specular In cases where the clutter does not fill the beam, the
Interference
region
region Constant-g region equation for volume uses the actual clutter extent in the
appropriate coordinate, weighted by the two-way antenna
20
gain. For example, if precipitation is visible over an altitude
10
0
20 log (r 0 /b 0 ) interval h to h in which antenna gain is constant,
m
Reflectivity s 0 , in dB 10 10 log g (For data on the volume reflectivity, h , and other properties
0
R q
t c
æ
(
c a ö h –
æ
h ) n ö
------------
0 --------
V =
m
è
ø
c
2 ø
L
è
p
20
v
30
of volume clutter, see atmospheric clutter, rain clutter,
snow clutter, and CHAFF). DKB
40
Ref.: Probert-Jones, J. R., “The Radar Equation in Meteorology,” Proc. 9th
50
Weather Radar Conf., Oct. 23–26, 1961; Doviak, 1984; Barton (1988),
pp. 133–137; Morchin (1993), p. 80.
60
0.1 1 10 100
Grazing angle y, in dB
weather clutter (see atmospheric clutter, rain clutter,
Figure C31 Typical surface clutter reflectivity vs. grazing
snow clutter).
angle.
COAST. The coast mode is a radar memory mode that causes
other properties of surface clutter, see land clutter and sea
the range- or angle-tracking system to continue to move at a
clutter). DKB
constant rate equal to that of a target that has been under
Ref.: Barton (1988), pp. 124–132.
track. It is used to prevent transfer of lock to a stronger target
Transmitted clutter is the reflection of sidebands of the crossing the position of the target being tracked, or to con-
transmitted carrier, which may lie outside the rejection notch tinue a track during a signal fade or momentary burst of inter-
of the clutter filter. When these components are reflected ference. SAL
from strong clutter, they may appear in the target filter pass- Ref.: IEEE (1990), p. 8.
band, obscuring the target. This effect is most pronounced in
COAXITRON. The coaxitron is a modification of a grid-
CW and high-PRF pulsed-doppler radars operating near the
controlled tube for high-power radar applications in the UHF
surface or within a cloud of precipitation or chaff, where the
band. The electrical interaction system and the RF input and
nearby clutter has great range advantage over the target. DKB
output circuits in the coaxitron are integrated within the vac-
Ref.: Skolnik (1990), pp. 14.3-14.7.
uum envelope, providing better performance and increased
tree clutter (see land clutter). bandwidth in comparison with conventional grid-controlled
tubes. An example is the RCA A15193 device, 0.76m long,
The clutter visibility factor is defined as “the predetection
0.42m diameter, 63.5-kg weight, operating at 406–450 MHz
signal-to-clutter ratio that provides stated probabilities of
with 1.5 MW peak power. SAL
detection and false alarm on a display.” The equivalent term
for radars using automatic detection is the clutter detectability Ref.: Skolnik (1980), p. 213; Gilmour (1986), p. 194.
factor. DKB CODE, CODING. A code is a plan for representing each of a
Ref.: IEEE (1993), p. 199. finite number of values or symbols as a particular arrange-
Volume clutter. When clutter sources are distributed homo- ment or sequence of discrete conditions or events. Coding is
geneously within a resolution cell of volume V , the clutter the corresponding process of transforming messages or sig-
c
RCS is given by nals in accordance with a definite code. In radar applications
there are two primary applications of codes: for generating
s = V h
c c v
modulated waveforms and for coding the information in radar
where
data transmission.
R q R q t c The main types of codes used in generating phase-coded
c a ö c e ö n ö
æ
æ
--------
-----------
V = æ ------------ ø L ø 2 ø
è
è
c
è
L
p p waveforms are binary (phase) codes (or biphase codes) using
only two sets of phase discretes (0 and 180° to modulate the
)
and h is the volume reflectivity, R is the range to the cell, q a carrier, and polyphase codes using smaller increments of
v
c
and q are the half-power beamwidths in azimuth and eleva- phase, in general N sets. An example of the first type is the
e
tion, L is the beamshape loss, t is the processed pulse Barker code, and examples of the second type are the Frank
p
n
width, and c is the velocity of light. The beamshape loss, L = code, P-codes, quadriphase codes. Both biphase and
p
2
1.33, is defined such that q q/L = pqq/(8 ln2) is the inte- polyphase codes can be formed by the use of complementary
p
a e
a e
grated solid angle within the two-way pattern of a Gaussian
codes to cancel the time sidelobes, as in the Golay (biphase)
beam (Probert-Jones, 1962; Doviak and Zrnic, 1984, p. 52).
and Welti (polyphase) codes. Codes can be used also for
Note that the factor p/4 relating the area of an ellipse to that
amplitude modulation of the waveform (e.g., Huffman code).
of the rectangle is included in this factor.